1. Field of the Invention
This invention relates to a semiconductor laser array device which has a built-in refraction index difference composed of the real number portion alone.
2. Description of the Prior Art
Semiconductor laser devices which are useful as light sources for information processing apparatus such as optical discs, laser beam printers etc., have been put into practical use. With an increase in information to be processed, quick information processing is required, and the development of semiconductor devices which can produce the highest possible output power but which can retain stabilized operation characteristics has begun. Even semiconductor lasers having a single active region, the development of which have been most advanced, can only produce an output power of approximately 50 mW at the maximum value in practical use.
On the other hand, phase-synchronized laser arrays (including gain guided laser arrays and index guided laser arrays) have been proposed in which a plurality of semiconductor laser devices are disposed in a parallel manner to attain optical phase coupling therebetween resulting in a laser device producing great output power with a single phase. However, in conventional gain guided laser arrays, loss of light increases with a decrease in a gain in the phase coupling region positioned between the adjacent laser operation areas, so that a phase shift of the electric field between the adjacent laser operation areas reaches 180.degree., resulting in the far-field patterns thereof having plural peaks, as shown in FIG. 4.
In order to improve the above-mentioned undesirable phenomena produced by conventional gain guided laser arrays, the use of index guided laser arrays can be recommended. The index guides laser arrays have been disclosed by the following articles:
A CSP-LOC (channeled-Substrate Plenar Large-Optical-Cavity) laser device proposed by D. Botez et al., of RCA Laboratories, Fourth International Conference on Integrated Optics and Optical Fiber Communication, Abstract 29B5-2, June 27-30, 1983 Tokyo, Japan has the disadvantage that the amount of laser light absorbed to the GaAs substrate varies depending upon the distance from the active layer to the mesa-portion of the substrate. If the distance between the active layer and the mesa-portion of the substrate were small, the amount of laser light absorbed to the GaAs substrate becomes great so that the laser device cannot produce high-output power. If the distance therebetween were great, the refractive index difference required for stable laser oscillation could not be created. Thus, a zero shift in the optical phase between the adjacent laser operation areas in unattainable.
A ridged laser array device proposed by D. E. Ackley et al., of Hewlett Packard Laboratories, Appln. Phys. Letters, 42(2), 15, p. 152, January 1983, has the disadvantage that laser light at the region between the adjacent laser operation areas is absorbed into the electrode to a great extent, resulting in a 180.degree. phase shift between the adjacent laser operation areas.
As a laser array other than the above-mentioned index guided laser arrays, a buried type laser array device has been proposed by D. E. Ackley et al., of Hewlett-Packard Laboratories, Appln. Phys. Letters, 39(1), p. 27, July 1981, which operates in leaky modes to result in a far-field pattern having two peaks, although it is excellent in the phase coupling efficiency between the adjacent laser operation areas.